1. Field of the Invention
The present invention relates to a focal plane shutter device including two sector groups (first and second blinds) disposed in the vicinity of an exposure window (opening: aperture) of a camera and each consisting of a plurality, viz., four or five pieces, of split sectors.
2. Related Background Art
In recent years, with increasing demands for hyperfine pictures and new expressions of pictures, it has been desired that a film sensitivity be improved, and a shutter speed of the camera be increased. Then, in response to the demand for the higher shutter speed, there was proposed a camera in which a shutter speed as high as 1/8000 sec is actually attained.
For such an increase in the shutter speed, the following construction was proposed. In a conventional focal plane shutter of such a type that an opening having a wide area is needed, a sector unit including two combined sector groups each consisting of a plurality of sectors is operated at a high speed by an electromagnetic driving source. For this purpose, the sectors composed mainly of carbon fibers are employed in place of light metals such as titanium, aluminium and beryllium.
Further, for preventing a direct impingement between the sector group having the sectors folded (superposed) and the sector group having the sectors that spread during an operation of the shutter among the plurality of sectors, these sector groups (first and second blinds) are disposed separately in front and in rear of a light shielding plate formed with an opening. The sectors are thus prevented from being damaged and vibrated. In this case, a spacing between these two sector groups is desirably widened in order to attain a smooth operation without causing a contact between those sector groups. Where the spacing between the two sector groups is simply widened, however, there exists a possibility in which the light leaks (light leakage) during the shutter operation when the sectors and the light shielding plate are largely spaced. Therefore, the first and second blinds should be disposed close to the light shielding plate at a distance close enough not to cause the light leakage. Then, an intermediate plate in addition to the light shielding plate is further interposed between the two sector groups, thus widening the spacing between the two sector groups while restraining the light leakage in the prior art. If a thickness of the light shielding plate is increased, the spacing between the two sector groups can be similarly widened while restraining the light leakage. For the following reason, however, there is taken the arrangement that the intermediate plate other than the light shielding plate is disposed.
In the split sectors of the respective sector groups, a quantity of vibrations increases with a larger amount of movement of the sectors. Hence, there is a possibility wherein the sectors having the large quantity of vibrations contact an edge (edge in the vertical direction) of the opening of the light shielding plate. In the case of the first blind, the sector having the largest moving quantity moves at a relatively low speed on the occasion of a release. The vibration quantity does not therefore become large enough to contact the edge of the opening. Further, the sector having the least moving quantity is disposed closest to the light shielding plate, and hence there is no such possibility as to contact the edge of the opening even when running the sector at a high speed. On the other hand, in the case of the second blind, the sector having the largest moving quantity when running the shutter at the high speed is disposed in a position close to the opening. A probability that this sector contacts the edge of the opening is high.
For avoiding this contact, it may be satisfactory that the edge is retreated from the sector by expanding the opening in the moving direction when running the sectors at the high speed. However, the opening incorporates a function to determine a field angle. It is therefore required that the opening be expanded only on the face side, confronting the second blind, of the light shielding plate. However, it requires considerable labor and is difficult to work the light shielding plate in that way. Thus, the spacing between the first blind and the second blind is widened by providing an intermediate plate 42 having an opening 42a that is more expanded in the moving direction during high-speed running of the sectors than an opening 30a of a light shielding plate 41 in addition to this light shielding plate 41 having the opening 30a for determining the field angle as shown in FIG. 1. Then, these light shielding plate and intermediate plate are incorporated as independent members into the focal plane shutter device.
In the thus constructed focal plane shutter device also, a much higher shutter speed exceeding 1/8000 sec has been recently demanded. It is therefore necessary to further reduce a weight of the sector members to be driven.
It is not, however, easy to reduce the weight of the sector members. A combination with a rigidity required of the sectors should be considered. Namely, when decreasing a thickness of the sector for the purpose of reducing the weight, the rigidity of the sector itself declines. There appears such a phenomenon that the sectors are undulated during a run or stop of the shutter. The undulation advances in the longitudinal direction of the sectors. Besides, the light shielding plate and the intermediate plate which partition the first and second blinds are vibrated due to an impact caused when the shutter stops.
When performing the next shutter operation in this state, the sectors, the light shielding plate and the intermediate plate contact each other, resulting in damage to the sectors themselves. It is predicted that particularly the light shielding plate and the intermediate plate which undergo the impact due to the stop of the first blind are repeatedly vibrated with such a flexure as to contact the second blind. If the second blind runs before the vibrations are attenuated, the sectors of the second blind are often broken because of an interference with the light shielding plate or the intermediate plate.
For this reason, in this type of focal plane shutter device, it is highly desired that the impact caused by the stop of the first blind is not transferred to the second blind.